Bound Water Content of Air‐Dry Soils Measured by Thermal Analysis

Abstract

Conventional oven-drying method may not give accurate information of bound water content in soils. The objective of this study was to determine the ftaction of soil water that is bound to soil colloids using the thetmogravimetry (TG) technique. A heating program from room temperature (RT) to 200°C was developed to partition bound water into loosely bound water (equivalent to water content determined by conventional oven-drying method) and tightly bound water that was not accounted by conventional oven-drying method on original and organic matter (OM) removed soil samples. Nine air-dry soils, with varying amounts of clay and OM contents, wete tested.For the original air-dry samples, bound water content ranged from 0.54 to 5.22%, and the fractions of loosely and tightly bound water were about 80 and 20%, respectively. On a mass fraction basis, the specific watet adsorption capacity o( soil OM was 10 to 40 times that of soil minerals. For most mineral soils, however, the contribution of soil minerals to bound water (>70%) was much larger than that of soil OM (<30%), since the mineral fraction usually dominates over the OM fraction. In a soil with high OM concentration but a relatively low clay content.OM contribution to bound water exceeded 50%. Soil specific surface area (5/i) showed a strong infiuence on bound water content, and a linear relationship between SÀ and bound water content was established.Abbreviations: DSC, difFerential scanning calorimetry; DTA, differential thermal analysis; EGD, evolved gas detection; MS, mass spectrometry; OM, organic matter; RT, room temperature; SA. specific surface area; SAR, specific adsorption ratio; TG, thermogravimetry.T he structure, energy, and properties (e.g., specific heat capacity and dielectric permittivity) of soil water vary with the distance from solid surface (Etzler, 1983; Yamaguchi, 1959). Water bounded in the first few layers is assumed to have stronger bonding energy (Panchev et al., 2005) and therefore different structure than the water in bulk (Bogdan et al., 1996;Kleinberg and Griffin, 2005). Soil water content is usually referred to the quantity of water that is evaporated from soil by heating the sample at 100 to 110°C (average 105°C) until there is no further weight loss. Some research has implied that the drying temperature of porous medium perhaps should be higher than 105°C. For example, thermo-analytical studies of water on activated alumina showed that the drying temperature for this material should be 120 to 125°C (Hampson and Bleam, 1996). Bogdan et al. (1996) measured water content of pyrogenic silica powder by drying at 200°C. Gardner et al. (2001) pointed out that heating at temperature range of 110 to 16O°C was required to remove bound water from clay surfaces. Logsdon and Laird (2004) dried clay samples at 190°C. Mitchell and Soga (2005) stated that depending on soil texture, the drying temperature for mineral soils should be in the range of 100 to 300°C. Thus it is questionable if the conventional method of oven-drying soil samples at 105...

Bound and mobile soil water isotope ratios are affected by soil texture and mineralogy, whereas extraction method influences their measurement

“…At higher temperatures, the matric potential effects on oxygen and hydrogen both diminish as hydrogen bonds between fine‐grained soil particles and bound water are more easily broken at high temperatures (Gaj et al, ). This supports other studies that found that bound water is more readily released at higher temperatures (Araguás‐Araguás et al, ; Gaj et al, ; Walker et al, ; Wang et al, ).…”

“…Our results showed that the water extracted by centrifuging falls on a mixing line between WaterE and WaterD (Figure ). We interpret this to mean that oven drying at 105°C does not remove all bound water from soils and, while this bound water is not removed, it also readily mixes with the presumably mobile water added subsequently (Wang, Lu, Ren, & Li, ). The contribution of WaterE to the centrifuged samples ranged from 0.21% to 2.24% based on hydrogen and 0.15% to 2.50% based on oxygen.…”

Application of thermogravimetric analysis for the evaluation of organic and inorganic carbon contents in agricultural soils

“…The inclusion of the clay content in addition to thermal mass losses in bivariate regressions was significant in the temperature intervals 110-420 and 200-370°C, but little improvement in the R 2 value was observed, whereas for all other studied intervals, the impact of the clay content was insignificant (Table 3). Some reasons for the weak relationship may be the fairly narrow range of SOC (0.38-5.04 %), the highly variable ratios between clay and SOC contents (1:1-110:1) indicating the different proportions between water fractions [46], sample compositions, and heterogeneities of soil organic compounds as soil samples derived from different soil types, bedrocks, land uses, and agricultural activity.…”

Influence of Loosely Bound Water on Compressibility of Compacted Fine‐Grained Soils

“…TBW has a strong viscosity and cannot be removed entirely from the surfaces of soil particles until the soil temperature is raised up to 200°C. Because of its unique properties, TBW is generally incorporated as a part of soil particles in calculations [14].…”